1. Field of the Invention
The present invention relates to an oxide superconducting wire, a method of preparing the same, and a method of handling the same, and more particularly, it relates to improvement for distortion resistance of critical current density in an oxide superconducting wire.
2. Description of the Background Art
In recent years, superconducting materials of ceramics, i.e., oxide superconducting materials, are watched as materials exhibiting higher critical temperatures. In particular, those of yttrium-based, bismuth-based and thallium-based, which exhibit high critical temperatures of about 90 K, 110 K and 120 K respectively, are expected as practicable superconducting materials.
In relation to a method for obtaining an oxide superconducting wire, such as a long superconducting wire or a superconducting pattern which is wired on a proper substrate, using a superconductor of such an oxide superconducting material, known is a method of covering raw material powder with a metal sheath and heat treating the same for bringing the raw material powder into a desired superconducting material, thereby preparing an oxide superconducting wire comprising a superconductor covered with the metal sheath. The as-formed superconducting wire is applicable to a cable, a bus bar, a current lead, a magnet, a coil or the like.
In order to apply the aforementioned superconducting wire to a cable, a magnet or the like, it is necessary to attain a high critical current density, in addition to a high critical temperature. In particular, it is necessary to ensure a critical current density required under a magnetic field which is applied to the superconducting wire, while such a high critical current density must be maintained under distortion which is applied to the superconducting wire.
However, a superconducting wire containing an oxide superconductor is so far inferior in distortion resistance of critical current density that the critical current density is reduced when the superconducting wire is bent at a certain curvature level, for example.
Accordingly, an object of the present invention is to provide an oxide superconducting wire, whose critical current density is not much reduced even if the same is distorted, and a method of preparing the same.
Another object of the present invention is to provide a preferable method of handling the aforementioned superconducting wire.
The inventive oxide superconducting wire, which has the so-called multicore structure, comprises a metal sheath and a plurality of superconductors independently distributed in the metal sheath along the thickness direction of the metal sheath. The thickness-directional dimension of each superconductor is set to be not more than 5% of the thickness-directional outside dimension of the metal sheath.
The present invention is particularly advantageously applied to a superconductor which is prepared from an oxide superconductor.
The present invention also provides a method of preparing a superconducting wire, which contains the aforementioned oxide superconductor. This method comprises a step of preparing a plurality of strands formed by oxide superconductors which are covered with first metal sheaths, a step of filling the plurality of strands into a second metal sheath, and a step of performing deformation processing for applying cross-sectionally a compressive load to the second metal sheath at least one time, being filled with the plurality of strands, so that the thickness of the superconductor contained in each strand is not more than 5% of the thickness-directional outside dimension of the second metal sheath and the second metal sheath is deformed into a tape.
The present invention further provides a method of handling an oxide superconducting wire. The oxide superconducting wire comprises a metal sheath having a thickness-directional dimension and a plurality of oxide superconductors which are independently distributed in the metal sheath in the thickness direction. The thickness-directional dimension of each oxide superconductor is set to be not more than 5% of the thickness-directional outside dimension of the metal sheath. In order to handle such an oxide superconducting wire, distortion (thickness of metal sheath/bend diameter) is controlled in a range of not more than 0.3%.
Once a superconductor is cracked, such cracking is easily propagated. This tendency is remarkable in an oxide superconductor of ceramics. When the superconductor is distorted in excess of a certain level, therefore, the superconductor is cracked and such cracking is propagated to portions having smaller distortion, to reduce the critical current density. According to the present invention, however, a superconductor is so divided that the thickness of each superconductor part is not more than a prescribed value, whereby it is possible to prevent propagation of cracking without reducing a flowable current value. Thus, distortion resistance of critical current density can be improved.
Thus, according to the present invention, it is possible to obtain a superconducting wire, whose critical current density is not much reduced even if the same is distorted. Therefore, it is possible to apply an oxide superconducting wire, which must have high distortion resistance in particular, to a significantly distorted element such as a cable or a magnet, for example, with no problem.
When an oxide superconductor is employed in the present invention, such an oxide superconductor is preferably c-axis oriented in the thickness direction.
The oxide superconductor may be any one of yttrium, bismuth and thallium, while a bismuth oxide superconductor is particularly optimum. In such a bismuth oxide superconductor, having components of Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O or (Bi,Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O, the a-b plane of a 2223 phase showing a critical temperature of 110 K, which has a 2223 composition of the components Bixe2x80x94 or (Bi,Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cu, is preferably oriented in a direction of current flow. According to the present invention, it is possible to easily attain such a structure that the a-b plane of the 2223 phase, showing a critical temperature of 110 K, is oriented in the direction of current flow.
The bismuth oxide superconductor is regarded as particularly preferable, also in a point that it satisfies all conditions of higher critical temperature and critical current density, small toxicity, and no requirement for rare earth elements as compared with yttrium and thallium oxide superconductors.
According to the present invention, it has been found possible to improve distortion resistance of critical current density also in relation to yttrium and thallium oxide superconductors, since degrees of orientation thereof can be improved to some extent.
The inventive superconducting wire may be shaped into a wire, so that the same is practicable in a wider range.
When the inventive superconducting wire, which is shaped into a wire, may be further covered with an organic coat of an organic material, superconducting properties of the superconducting wire can be further stabilized against bending.
According to the inventive method of preparing an oxide superconducting wire, it is easy to divide the thickness of an oxide superconductor, which is covered with a metal sheath, so that the thickness of each superconductor part is not more than 5% of the overall thickness of the superconducting wire.
The strands may be drawn before the same are filled into the second metal sheath, in order to effectively reduce the thickness of each superconductor contained in the as-formed oxide superconducting wire.
The second metal sheath is preferably deformed into a flat tape by deformation processing which is applied in the inventive preparation method.
In order to improve the critical current density, heat treatment is preferably performed after the deformation processing. Further, it is more preferable to repeat such deformation processing and heat treatment a plurality of times.
It is possible to easily adjust the thickness of each superconductor contained in the oxide superconducting wire obtained in the step of deformation processing, by varying the number of strands which are filled into the second metal sheath. For example, the thickness of each superconductor can be easily reduced after deformation processing, by increasing the number of the strands.
According to the present invention, further, it is possible to substantially prevent deterioration of superconducting properties resulting from distortion, by controlling the distortion, which is applied to the superconducting wire containing oxide superconductors, in a range of not more than 0.3%, as hereinabove described. Such distortion control is performed in various steps, including heat treatment, for example, for preparing the superconducting wire, and processes of delivery from a reel after preparation, winding on the reel, and shaping into a cable, a bus bar, a power lead, a coil, or the like. When the distortion is thus controlled in a range of not more than 0.3%, the critical current density is not substantially reduced even if the superconducting wire is repeatedly distorted. Thus, it is possible to apply the superconducting wire to various uses.
When the superconducting wire is heat treated in a state wound at a curvature level applying distortion of not more than 0.3%, the superconducting properties are not in the least or hardly deteriorated during heat treatment and delivery. Thus, it is possible to obtain a superconducting wire which shows stable superconducting properties particularly against bending.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.